The present invention relates to a tilt angle detecting device including a tilt sensor, which has two types of liquids, each having different properties, as sealed in a transparent container, and which detects tilting by detecting a relative movement between the liquids.
As means for detecting a tilt of an instrument or a device, an air bubble tube is known, and the air bubble tube is a glass container with a liquid and an air bubble sealed in. In a case where an instrument or a device is to be installed for a case where horizontality is required such as a surveying instrument, a tilt angle detecting device provided with the air bubble tube is used. As the air bubble tube, a photo-electric air bubble tube is known, which detects the movement of air bubbles in the air bubble tube photo-electrically. By referring to FIG. 7 to FIG. 9, description will be given below on a conventional type tilt angle detecting device having the photo-electric air bubble tube.
A photo-electric air bubble tube 1 comprises a light source 2, photodetection elements 3 and 4, and an air bubble tube 5 in approximately cylindrical shape, and a tilt angle is detected based on signals from the photo-electric air bubble tube 1.
The light source 2 is disposed below the air bubble tube 5, and an optical axis of the light source 2 perpendicularly crosses a center line of the air bubble tube 5. A pair of the photodetection elements 3 and 4 are positioned opposite to the light source 2 with the air bubble tube 5 between them, and are disposed at such positions symmetrical with respect to the optical axis of the light source 2 and at positions separated by a distance as required from each other. As the photodetection elements 3 and 4, photo-electric conversion elements are used.
As shown in FIG. 7, in a direction passing through the center line, a detection light 7 advances approximately in a straightforward direction and passes through air bubbles 8 and a liquid 9 in the air bubble tube 5 and are received by the photodetection elements 3 and 4. In boundary portion between the air bubbles 8 and the liquid 9, the detection light 7 is reflected and the detection light 7 is not detected by the photodetection elements 3 and 4.
FIG. 8 is a cross-sectional view of a portion where the air bubbles 8 of the air bubble tube 5 exists, and FIG. 9 is a cross-sectional view of portion of the liquid 9 where the air bubbles 8 of the air bubble tube 5 do not exist. As shown in FIG. 8, in a case where the air bubbles 8 exist, the detection light 7 straightly projected through the center of the air bubbles 8 is received by the photodetection elements 3 and 4. Also, the detection light 7 passing through the liquid 9 around the air bubbles 8 is received by the photodetection elements 3 and 4 due to an optical action. Further, upon reaching a boundary portion between the air bubbles 8 and the liquid 9, the detection light 7 is reflected by a boundary surface, and the detection light 7 is not received by the photodetection elements 3 and 4. Therefore, differences occur in the light amount, which are received by the photodetection elements 3 and 4 in such a degree as reflected by the boundary surface, between the portion where the air bubbles 8 exist and the portion where the air bubbles 8 do not exist, as shown in FIG. 9.
Therefore, the light amount of the detection light 7 entering the photodetection elements 3 and 4 varies according to the position of the air bubbles 8. Since the position of the air bubbles 8 varies according to the tilting, the tilt angle can be detected by detecting the difference of photodetection amount between the photodetection element 3 and the photodetection element 4.
However, in the conventional type tilt angle detecting device, there have been problems in that a portion of the detection light 7, which passed through the air bubbles 8, is detected by the photodetection elements 3 and 4 as steady state light noise and the S/N ratio is decreased.
Also, in the conventional type tilt angle detecting device, since a dynamic range of the photo-electric type air bubble tube 1 is smaller, it is difficult to secure detection accuracy. Further, fine adjustment is needed for the setting when the center line of the air bubble tube 5 is set to perpendicularly cross the optical axis of the detection light 7, and an adjusting mechanism is further provided. As a result, there have been problems in that the structure becomes more complicated.